Record your own music excellent problems encountered Mall

1. Add parameters have constructed forget to add a constructor with no arguments

  ## 错误提示
  (no Creators, like default construct, exist ): cannot deserialize from Object value (no delegate- or property-based Creator)

2. Forget injection @Autowired

  ## 错误提示
  Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause
  java.lang.NullPointerException: null